API serializer foreign count fields do not have a default value

[markkuleinio]

Environment

• Python version: 3.7.3
• NetBox version: 2.9.11

Steps to Reproduce

1. Create a new tag with any name, like "Testing"
2. Get the same tag via API and see tagged_items field

Expected Behavior

tagged_items is 0 (API docs say the field is integer)

Observed Behavior

tagged_items is null

$ curl -s -X GET "http://netbox-test.example.net/api/extras/tags/?name=Testing" -H "Authorization: token xxx" | jq .
{
  "count": 1,
  "next": null,
  "previous": null,
  "results": [
    {
      "id": 4,
      "url": "http://netbox-test.example.net/api/extras/tags/4/",
      "name": "Testing",
      "slug": "testing",
      "color": "9e9e9e",
      "description": "",
      "tagged_items": null
    }
  ]
}

[jeremystretch]

This happens in other areas, as well, when there are no related objects. Need to force null to zero somehow.

[DanSheps]

Looks like we can do a default=0 in the serializer. This could be breaking though, perhaps we want to wait for 2.11?

[jeremystretch]

I'd argue that it's really fixing a bug rather than changing previously-desired behavior, so I think it's okay to change now.

[DanSheps]

I am going to change all instances of *_count = serializer.IntegerField and count_* = serializer.IntegerField to have default=0 in addition to this tagged_items field.